Commercial portable radio devices, such as mobile phones, include some foldable, i.e. clamshell models. These have got two parts such that the parts can be folded over, on a hinge, so that they lie on top of each other or adjacently end-to-end in almost the same plane. In the first, closed-up, position, the device is particularly small, and in the latter, opened, position the device is used during communication.
Antennas used in foldable mobile phones are normally monopole-type external antennas. Their drawback is the impracticality generally associated with a protruding structural element. Naturally it would be possible to use internal PIFA-type planar antennas, but the thin structure of the folding parts in the mobile phone would result in the distance between the radiating plane and ground plane to be so small that the antenna gain would be unsatisfactory. Furthermore, it would be possible to have an internal monopole-type planar antenna such that the radiating plane would not be located face to face with the ground plane. In that case the thinness of the device would cause no problem as such, but the electrical characteristics such as matching and antenna gain would again be unsatisfactory. Matching could be improved using an additional circuit, but this would require the use of several discrete components.